A plurality of different anesthetics are commonly used in combination in modern anesthesia technique. Adjusting the dosages of the individual anesthetics and thus controlling the course of anesthesia is the task of the anesthesiologist. Volatile and intravenously dispensable hypnotics as well as intravenously dispensable opioids are available. Common practice is, furthermore, the combination of volatile and intravenous anesthetics as well as the additional administration of nitrous oxide. The effect of the combined anesthetics does not, as a rule, correspond to the sum of the individual effects of the individual anesthetics, but synergistic interactions occur. This makes the adjusting of the dosages of the individual anesthetics a complex task, which can be supported by a clear graphic representation of the concentration-vs.-effect relationship.
The concentrations of the anesthetics at the site of action (usually the brain) can be calculated by means of so-called pharmacokinetic compartment models from the quantities of said anesthetics fed per unit of time. Based on the concentrations of the active ingredients at the site of action, an effect can be estimated on the basis of common pharmacodynamic interaction models. The interaction of a plurality of anesthetics leads to an anesthesia effect, which can be described as a probability that a defined pain stimulus (e.g., laryngoscopy or skin incision) is tolerated. It is postulated in an article by T. Boullion et al. (Anesthesiology, 2004; 100; 1353-1372) that different combinations of hypnotics and opioids lead to equal anesthesia effects. If the concentration at the site of action is plotted on the y-axis, that of the opioid on the x-axis and the probability that a certain stimulus is suppressed on the z-axis, an effect or response surface is obtained for the probability of the tolerance of a stimulus (e.g., laryngoscopy or skin incision). The contour lines of the response surface, i.e., sections through the response surface in parallel to the x-y plane, yield lines of equal anesthesia effect. These lines of equal effect are called isoboles.
A system for feeding at least one first anesthetic and at least one second anesthetic in a such a way that the quantity being fed is controlled in a quantitatively adjustable manner as well as for displaying the action diagram is described in DE 10 2004 050 717 B3. The x-y coordinate plane with the concentrations of the two anesthetics is shown on the two axes. A response surface determined in advance is superimposed to this coordinate plane, and the chronological sequence of the concentration data of the previous course of anesthesia is displaced as a trajectory in the x-y plane. Furthermore, DE 10 2007 038 975 A1 describes the calculation of an anesthesia effect (NSRI), which expresses the synergistic effect by a numerical value. The NSRI value is constant along isoboles.
The above-described interaction model has been developed for purely intravenous anesthesia, i.e., for a first intravenous anesthetic and a second intravenous anesthetic, and it was later extrapolated to purely volatile anesthesia. New concentration-vs.-effect relationships, which depend on the individual concentrations and the ratios of these concentrations at the site of action, arise in case of balanced anesthesia, in which an intravenously administered hypnotic (e.g., propofol) and volatile anesthetics (a hypnotic, e.g., sevoflurane, and an opioid, e.g., remifentanyl) are used.